1. Field of the Invention
The present invention relates to a boundary acoustic wave device utilizing a boundary acoustic wave which propagates along a boundary between a first medium layer and a second medium layer having a different sound velocity from the first medium layer, and more particularly, to a boundary acoustic wave device which suppresses unwanted spurious signals.
2. Description of the Related Art
In surface acoustic wave devices utilizing a surface acoustic wave, such as a Rayleigh wave or a first leakage wave, reduced size and weight can be achieved, and in addition, adjustment is not required.
Thus, surface acoustic wave devices have been widely used for RF or IF filters in, for example, mobile phones, VCO resonators, and VIF filters for televisions.
However, since surface acoustic waves propagate along a surface of a medium, surface acoustic waves are sensitive to changes in the surface condition of the medium. Accordingly, in a chip in which surface acoustic waves propagates, a chip surface along which a surface acoustic wave propagates must be protected. Thus, a surface acoustic wave device must be hermetically sealed using a package having a cavity portion therein, such that the chip surface of the surface acoustic wave chip faces the cavity portion. As a result, the cost of the package described above is relatively high. In addition, the size of the package must be larger than the size of the surface acoustic wave chip.
A boundary acoustic wave device, which does not require the package having a cavity portion as described above, has been proposed.
FIG. 15 is a front cross-sectional view of a conventional boundary acoustic wave device. In a boundary acoustic wave device 101, a first medium layer 102 and a second medium layer 103 having different sound velocities are laminated to each other. At a boundary A between the first medium layer 102 and the second medium layer 103, an IDT 104 defining an electroacoustic transducer is disposed. In addition, reflectors (not shown) are disposed at the two sides of the IDT 104 in the direction along which a boundary acoustic wave propagates.
In the boundary acoustic wave device 101, by applying an input signal to the IDT 104, a boundary acoustic wave is generated. The boundary acoustic wave propagates along the boundary A of the boundary acoustic wave device 101, as schematically shown by arrow B in FIG. 15.
In “Piezoelectric Acoustic Boundary Waves Propagating Along the Interface Between SiO2 and LiTaO3” IEEE Trans. Sonics and Ultrason., VOL. SU-25, No. 6, 1978 IEEE, one example of a boundary acoustic wave device as described above is disclosed. In this device, an IDT is formed on a 126° rotated Y plate X propagating LiTaO3 substrate, and a SiO2 film having a desired thickness is formed on the LiTaO3 substrate so as to cover the IDT. In this structure, an SV+P type boundary acoustic wave (Stoneley wave) propagates. “Piezoelectric Acoustic Boundary Waves Propagating Along the Interface Between SiO2 and LiTaO3” IEEE Trans. Sonics and Ultrason., VOL. SU-25, No. 6, 1978 IEEE, discloses that when the thickness of the SiO2 film is set to 1.0 λ (λ indicates the wavelength of a boundary acoustic wave), an electromechanical coefficient of 2% is obtained.
In addition, in “Highly Piezoelectric Boundary Acoustic Wave Propagating in Si/SiO2/LiNbO3 Structure” (26th EM symposium, May 1997, pp. 53 to 58), an SH type boundary acoustic wave propagates in a [001]—Si<110>/SiO2/Y-cut X propagating LiNbO3 structure. This SH type boundary acoustic wave has an advantage in that an electromechanical coefficient k2 is increased as compared to that of the Stoneley wave. In addition, since the SH type boundary acoustic wave is an SH type wave, the reflection coefficient of electrode fingers defining an IDT reflector is increased as compared to that of the Stoneley wave. Thus, when a resonator or a resonator type filter utilizes the SH type boundary acoustic wave, greater miniaturization can be achieved. In addition, steeper frequency properties are obtained.
Since the boundary acoustic wave devices utilize boundary acoustic waves, which are disclosed in “Piezoelectric Acoustic Boundary Waves Propagating Along the Interface Between SiO2 and LiTaO3” IEEE Trans. Sonics and ultrason., VOL. SU-25, No. 6, 1978 IEEE and “Highly Piezoelectric Boundary Acoustic Wave Propagating in Si/SiO2/LiNbO3 Structure” (26th EM symposium, May 1997, pp. 53 to 58), a package including a cavity portion is not required. Therefore, the size and cost of the acoustic wave device are reduced. However, the inventors of the present invention have discovered that, when the boundary acoustic wave device is actually produced, unwanted spurious signals are often generated.
FIGS. 16 and 17 are views illustrating a problem with a conventional boundary acoustic wave device. FIG. 16 is a schematic perspective view showing the appearance of the boundary acoustic wave device 111, and FIG. 17 is a view showing the frequency properties thereof.
As shown in FIG. 16, on a Y-cut X propagating single crystal LiNbO3 substrate 112, an IDT 113 and reflectors 114 and 115 are formed using an Au film having a thickness of about 6.05 λ. In addition, on the single crystal LiNbO3 substrate 112, a SiO2 film 116 having a thickness of about 3.3 λ is formed by RF magnetron sputtering at a wafer heating temperature of about 200° C. so as to cover the IDT 113 and the reflectors 114 and 115. The number of electrode finger pairs of the IDT 113, the cross width, and the duty ratio of the electrode finger are set to 50 pairs, about 30 λ and about 0.6, respectively. In addition, the number of electrode fingers of the reflectors 114 and 115 are each set to 50, and the wavelength λ of the reflectors 114 and 115 is set to be substantially the same as the wavelength λ of the IDT 113. In addition, the distances between the center of the electrode finger of the IDT 113 and that of the reflectors 114 and 115 are each set to about 0.5 λ. On the upper and the lower sides of the Au film, thin Ti layers are formed by deposition in order to enhance the adhesion.
The frequency properties of a boundary acoustic wave device 111 formed as described above are shown in FIG. 17. As shown in FIG. 17, in the boundary acoustic wave device 111, spurious signals are generated at a higher frequency side which have greater intensities than the spurious signals generated at an anti-resonance frequency and the vicinity thereof.
Accordingly, when the boundary acoustic wave device 111 is used as a resonator, unnecessary resonance is generated by the spurious signals described above. In addition, when the boundary acoustic wave device 111 is used as a filter, the out-of-band suppression level is degraded thereby. Therefore, the spurious signals significantly interfere with the production of practical boundary acoustic wave devices.